1. Field of the Invention
The present invention relates to a vehicle regenerative braking apparatus.
2. Description of the Related Art
In recent years, a method for reclaiming or regenerating the deceleration energy of a vehicle by means of an electric machine has come into the stage of practical application with the goal of improving vehicle fuel efficiency. It is normal with vehicle regeneration methods to generate a braking force equivalent to the regenerative power (also called electrical braking force), and to make up the remaining required braking force with frictional force of the mechanical brake.
However, during regeneration, if operation of a large power-consuming load (e.g., a PTC heater) is stopped during regeneration, the share of regenerative power generated by the electric machine directed to battery charging increases. Therefore, the amount of power that can be accepted by the battery (maximum charge amount) is exceeded, and there is the possibility of detrimental effects to the battery. Further, the maximum charge amount is a function of multiple variables including the state of charge of the battery (SOC) and battery temperature. In instances such as when the SOC is near a state of full charge and the battery temperature is extremely high, the charge amount should be decreased, as compared to other instances.
As a solution to the aforementioned regenerative braking problem of regenerative power exceeding the maximum charge amount, it is conceivable to reduce the amount of regenerative energy at the point when the battery can no longer accept power, that is, at the point where regenerative power exceeds the maximum charge amount, in other words, when excess power (regenerative power−maximum charge amount) has been generated. However, if regenerative power is suddenly reduced then vehicle braking force (electrical braking force+frictional braking force) is lost and degrades the smoothness of driving as perceived and felt by the driver.
Even when compensation of the lost portion of electrical braking force by an increase in the frictional braking force of the hydraulic brake mechanism is attempted, it is difficult to make the speed of increase of frictional braking force from the hydraulic brake system match the dramatic speed of loss of regenerative power (several milliseconds). The following alternatives were thus conceived in the related art.
According to the related art disclosed in Japanese Patent No. 3,232,823, the efficiency of a generator is degraded when excess power is generated, and the excess power is converted to heat within the generator. The heat is then removed by generator absorption technology. However, when excess power is large it is very difficult to completely consume with this generator absorption technology, depending on how much the efficiency of the generator has been degraded. When using a component dedicated to power generation as an electric machine, adoption is physically impossible.
Japanese Patent Laid-Open Publication No. Hei 8-154304 discloses a switch-controlled discharge resistor which absorbs the excess power. However, with this discharge resistor technology, not only is a costly power resistor necessary, but a switching control device to set the power to be consumed by the discharge resistor to a value corresponding to the amount of excess power is also necessary. This requires increased production cost and increased installation space.
According to the related art disclosed in Japanese Patent Laid-Open Publication No. Hei 5-276686, excess power is directed into a catalytic heater or similar component. However, this entails only adapting the aforementioned PTC heater used as a load for consuming excess power for use as a catalytic heater, and when power is supplied to the catalytic heater at the point when excess power is generated, further consumption of excess power is difficult.